1. Field of the Invention
The present invention relates to a regenerable catalyst composition suitable for entrapping SOx. More particularly, the present invention relates to a Diesel Oxidation Catalyst (“DOC”) or a Catalyzed Soot Filter (“CSF”) comprising the catalyst composition of the invention. The invention further relates to the use of a catalyst composition that adsorbs SOx as metal sulfate under lean (oxidative) conditions and desorbs accumulated sulfate as SO2 under rich (reducing) conditions.
2. Background Art
Sulfur oxides (SOx) have a detrimental affect on the performance of automotive catalysts and traps. Such automotive catalysts and traps typically include noble metals and/or Ce-Zr oxide. At present, one of the most challenging problems in the design of lean burn and diesel engines relates to the removal of NOx components. Currently, there is a lack of known reductants for the selective catalytic reduction of NOx to nitrogen for such engines. However, current 3-way catalysts do meet the requirements of newly developed engines with lean air/fuel mixtures. NOx traps (NOx storage catalyst) have the ability to store NOx under oxidizing (lean) conditions and to reduce the stored NOx to N2 under reducing (rich) conditions. Unfortunately, a drawback of NOx traps is their intolerance to SOx compounds derived from sulfur in the fuel and lube oil, leading to a gradual deterioration of their performance (see, S. Hodjati, P. Bernhardt, C. Petit, V. Pitchon, A. Kiennenmann, Removal of NOx: Part I. Sorption/desorption processes on barium aluminate, Applied Catalysis B. Environmental 19 (1998) 209-219). This reference in its entirety is hereby incorporated by reference. Thermodynamically, metal sulfates decompose at higher temperatures and are more stable than the corresponding metal nitrates. It is difficult to create a NOx trap having sufficient sulfur resistance. This problem is especially challenging for diesel engines due to the higher sulfur level in diesel fuel than in gasoline. NOx traps currently are limited to markets where the sulfur content in gasoline and diesel fuel is very low. However, even with low sulfur fuel, periodic desulfation of the NOx trap is required.
One current strategy utilized to deal with the problems associated with desulfation is to increase the operating temperature of the catalyst to 600°-650° C. under rich conditions. High temperature operation using excess fuel leads to the gradual thermal deactivation of the NOx trap and requires a special control management strategy. Another possible solution for avoiding sulfur poisoning of a NOx trap or a DOC is to place a SOx storage material upstream of NOx trap. Such a SOx storage material should be able to collect SO2 under lean conditions in the operating temperature window of the NOx trap (normally 300-450° C.). The SOx storage material may be regenerated under conditions that are safe for the NOx trap. Under rich (reducing conditions) SO2 is not expected to be a poison for the NOx trap. This is particularly true because of the elevated temperatures during rich operation that are favorable for SO2 desorption. Moreover, the NOx trap is typically full of adsorbed NOx at the time of SO2 release thereby preventing SO2 adsorption on NOx trap material.
Materials that have been proposed for reversible SOx removal from flue and other industrial waste gases include copper, iron, and manganese containing systems. Studies have demonstrated that copper oxide-based sorbents (typically, 5% Cu on a support) have the best sorption-regeneration characteristics for applications at around 350-400° C. (see for example, R. F. Vogel, B. R. Mitchell, F. E. Massoth Reactivity of SO2 with supported metal oxide-alumina sorbents, Environ. Sci. Technol., 8, No. 5 (1974) 432-436; M. H. Cho, W. K. Lee, SO2 removal by CuO on γ-alumina, J. Chem. Eng. Japan, 16, No. 2 (1983)127-131; J. H. A. Kiel, W. Prins, W. P. M. van Swaaij, Flue gas desulfurization in a gas-solid trickle flow reactor with a regenerable sorbent, Gas Separation Technology (ed. E. F. Vansant, R. Dewolfs), Elsevier, Amsterdam, 1991, 539-548). Copper containing systems display reasonable stability in multi-cycle processes, including tolerance to water vapors and over-heating. These systems are currently used for high-temperature SOx removal from flue gases. Examples of such systems include Cu/Al2O3 and more recently Cu—CeO2 (Yoo K. etc. Ind. Eng. Chem. Res., v. 33, 7 (1994), p. 1786, J. F. Akyurtlu, A. Akyurtlu, Chem. Eng. Sci., 54 (15-16) 2191-2197 (1999), H. W. Pennline, Fuel & Energy Abstracts, 38 (1997), N3, p. 187, Centi G, Perothoner S., Developments in Chem. Eng. & Mineral Processing, 8 (2000), N5-6, p. 441, Wang Z. Industrial & Eng. Chem. Research, 37 (1998), N12, p. 4675, Jeong S., Kim S., Industrial and Eng. Chem. Research, 39 (200), N6, p. 1911).
More recently, a number of other materials have been proposed for SOx removal. Such materials include, Pt—CeO2—ZrO2 and Pt—CeO2 (F. M. Allen, S. Khairulin, T. J. Zega, R. J. Farrauto, Reusable SOx traps: Materials and methods for regeneration, AlChE Meeting, Nov. 15-20, 1998, Miami, Fla.; Section 4-3, p. 84-5), MgAl2-xFexO4 (J. Wang, Z. Zhu, C. Li, Pathway of the cycle between the oxidative adsorption of SO2 and the reductive decomposition of sulfate on the MgAl2-xFexO4 catalyst, J. Mol. Catal., 139 (1999) 31-41), MgAl2O4 (M. Waqif, O. Saur, J. C. Lavalley, Y. Wang, B. Morrow, Evaluation of magnesium aluminate spinel as a sulfur dioxide transfer catalyst, Appl. Catal., 71 (1991) 319-331), Co—Mg—Al mixed oxides (A. E. Palomares, J. M. Lopez-Nieto, F. J. Lazaro, A. Lopez, A. Corma, Reactivity in the removal of SO2 and NOx on Co/Mg/Al mixed oxides derived from hydrotalcites, Appl. Catal. B., 20 (1999) 257-266), and Cu—CeO2 (J. F. Akyurtlu, A. Akyurtlu, Chem. Eng. Sci., 54 (15-16) 2191-2197 (1999)). Moreover, dual-functional systems containing components for oxidizing SO2 to SO3 have been described. Such systems include Pt and SOx storing components selected from Ti, Zr, Sn, Fe, Ni, Ag and Zn oxides are described (K. Okuide, O. Kuroda, T. Yamashita, R. Doi, T. Ogawa, M. Fujitani, H. Lizuka, Sh. Azukibata, Yu. Kitahara and N. Shinotsuka. Jpn. Kokai Tokyo Koho JP 11 169, 708 (99 169,708) (Cl. BO1J23/42), 29 Jun. 1999, Appl. 1997/344,682, 15 Dec. 1997). These systems operate in two periodic steps that consist of an operation under oxidizing conditions and a far shorter operation under reducing conditions.
A number of other prior art systems are known for the removal of SOx. For example, the possible use of Pt—CeO2—ZrO2 and Pt—CeO2 in automotive applications has also been considered. (F. M. Allen, S. Khairulin, T. J. Zega, R. J. Farrauto, Reusable SOx traps: Materials and methods for regeneration, AlChE Meeting, Nov. 15-20, 1998, Miami, Fla.; Section 4-3, p. 84-5) The utilization of Pd—Ba sulfur traps has been attempted with only partial success. (Automotive Engineering/February 1997, p. 133). Ag/Al203 has been discussed as an effective SOx trap material (T. Nakatsuji, R. Yasukawa, K. Tabata etc. Highly durable NOx reduction system. SAE 980932). Finally, a number of prior art publications discuss the testing of sulfur traps without complete disclosure of the compositions. (O. H. Bailey, D. Dou and M. Moliner, Sulfur traps for NOx adsorbers, SAE 2000-01-1205; SAE 2000-01-1012; 2000-01-1932; 1999-01-2890; 1999-01-3557)
A number of U.S. patents also disclose SOx removing systems. For example, U.S. Pat. No. 5,472,673 (the '673 patent) discloses SOx adsorbents selected from alkali, alkali-earth, rare earth metals, that also contain Pt. The compositions disclosed in the '673 patent cannot be regenerated under NOx trap temperature limits thereby requiring a separate mode of operation under high temperatures. The materials of the '673 patent only function as irreversible SOx traps. Moreover, Pt containing adsorbents are also not acceptable due to the H2S release under rich conditions, as one can see below. U.S. Pat. No. 5,473,890 (the '890 patent) discloses a SOx absorbent containing at least one member selected from copper, iron, manganese, nickel, sodium, tin, titanium, lithium and titania. Pt is also used as a SOx adsorbent. The '890 patent does not disclose any example that illustrates the performance of such absorbents. The carrier utilized in the '890 patent is made of alumina with the adsorbent preferably being lithium. U.S. Pat. No. 5,687,565 (the '565 patent) discloses a very complex oxide composition this is designed for gasoline applications with high temperature of regeneration of SOx trap material. The composition of the '565 patent is an irreversible trap material because it is not designed to prevent the poisoning of NOx trap. The composition of the '565 patent contains alkaline-earth oxides (Mg, Ca, Sr, Ba) or Zn. The '565 patent mentions that Cu may be used to promote the basic formulations. Finally, the '565 patent discloses compositions that only use a small amount of noble metals (Ru, Os, Pd, Pt etc.). U.S. Pat. No. 5,792,436 (the '436 patent) discloses sorbents containing alkaline earth metal oxides of Mg, Ca, Sr, Ba in combination with oxides of cerium, Pr and group of oxides of elements of atomic numbers from 22 to 29 inclusive. Pt is used in each of the adsorbents of the '436 patent. The regeneration temperatures for SOx removal are high for the compositions of the '436 patent.
Although a number of prior art systems are known for the removal of SOx as set forth above, many of these systems are used for stationary systems and/or industrial systems (i.e., manufacturing plants). Accordingly, there is a need for alternative SOx removal systems for automotive applications.